The gold standard for the measurement of respiratory air flow is the pneumotachograph, which determines flow by measuring differential pressure across a fine screen in the flow stream. Pneumotachographs are widely used but limited to 1) short term measurements of ventilation in respiratory medicine and exercise testing, and 2) long term measurements in intubated and unconscious patients who are dependent on a conventional ventilator. The pneumotachograph is rarely used for long term measurements of ventilation in non-intubated, conscious patients and during sleep due to the weight of the pneumotachograph and the discomfort of the pneumotachograph and a mask and headgear that must be worn. However, there is a tremendous need to monitor ventilation in several medical disciplines, particularly during sleep, as sleep unmasks several respiratory disorders, which remain undetected by short term measurements of ventilation during wakefulness.
Sleep induces changes in the respiratory system that affect the central control of either ventilation or upper airway patency. As a consequence, patients may exhibit sleep related breathing disorders that show a clinical spectrum from obstructive sleep apnea to severe hypoventilation in patients with co-morbid cardiopulmonary and neurological disorders. Although the impact of sleep on ventilation is well recognized, quantitative measurement of ventilation has not been well established in clinical diagnostics. This is in part due to the inability to monitor air flow during longer time periods. Instead, less valuable qualitative measures of air flow using a nasal cannula or thermistors are typically performed for diagnosing sleep disordered breathing. Nasal cannulas, nasal prongs, thermistor-based devices, and other qualitative air flow measurement devices are used regularly for overnight sleep studies because they are significantly more comfortable for the patient than pneumotachographs. These semi-quantitative measures of respiration are used to detect apneas and hypopneas by correlating relative changes in the measurement but they are neither suited to quantify the degree of upper airway function (e.g., but not limited to peak inspiratory air flow, inspiratory and expiratory resistance) nor to quantify the level of ventilatory impairment (e.g., but not limited to tidal volume and minute ventilation). Moreover, recent studies demonstrate, that using a quantitative air flow measurement for sleep studies will also improve the detection of mild degrees of sleep related breathing disorders, which remain undetected by the current state of measuring ventilation during sleep. This matter is particularly important in children in which mild degrees of upper airway obstruction (snoring) leads to substantial daytime impairments. Likewise, subtle respiratory disturbances during sleep may trigger adverse cardiovascular events in adults with co-morbid conditions. In summary, the lack of a high quality, quantitative air flow measurement during sleep studies represents a significant loss of valuable clinical information that limits diagnostic accuracy and clinical care. In particular, quantitative flow measurement would allow clinicians to directly measure key respiratory parameters such as breath-by-breath tidal volume, inspiratory flow rates, and timing indices over prolonged periods of time.
Particularly in children, investigators have established that even mild disturbances of breathing during sleep impact academic performance and predict the presence of attention deficit disorders in children. Moreover, early detection of these sleep related breathing disturbances and consequently treatment of these disturbances has been shown to improve these adverse effects.
Currently, no device exists for obtaining a quantitative measurement of respiration during sleep, without disturbing the patient. The pneumotachograph is too cumbersome for use during sleep and the nasal cannula or thermistor only provides a qualitative but not quantitative measure of ventilation.
In summary, particularly in sleep medicine, there is a need for a device that is both comfortable and provides a quantitative, accurate measure of air flow for early detection of sleep related breathing disturbances.